Named after the mythological beast slain by the Greek demigod Heracles, the Hydra is a genus of freshwater animal related to jellyfish and sea anemones, best known for its ability to regenerate whole individuals from parts of another. And until recently, the way it opened its mouth—the animal's only orifice—was a mystery.
Hydra must stretch a hole through its epithelial tissue (skin) to make an opening to its mouth. The biomechanics of how that worked at the cellular level was previously unknown. Some biologists had theorized that it had to rearrange the positions of some of its cells (think of a Rubik's cube) to create and then expand its mouth opening.
“This process happens so quick [within one minute, according to the new research]; this was the first indication to us that mouth opening did not involve cellular rearrangements,” UC San Diego Assistant Professor of Biology and Physics Eva-Maria S. Collins said in a March 8 UC San Diego press release. The study, headed by Collins, was published in the March 8 edition of the Biophysical Journal.
A team of biologists and physicists at two University of California schools, UC San Diego and UC Irvine debunked the cellular rearrangement hypothesis, showing that—while they're mostly indistinguishable from the cells around them—it's the same set of cells that consistently open and close to create the mouth. They discovered that the process occurs when the organism contracts muscle-like fibers in some of its endothelial cells (the inner layer) to open its "lips".
“...this work is exciting [because] there are very few systems in which you can do quantitative measurements in vivo,” Collins said. “Hydra is such a simple organism; it allows us to perform controlled perturbations and quantitative measurements in the natural context.”
To conduct their study, the research team utilized live imaging, biopharmacology, and shape analysis, which uses statistical analysis of geometric shapes to do object recognition and matching.
The hydra utilized in the study were genetically modified, or transgenic. They had their epithelial cells marked with fluorescent-sensitive proteins to allow scientists to track specific cells as they moved around in the living (in vivo) animal. Their ectodermal (the outer layer) epithelial cells glowed green and their endodermal epithelial cells glowed red. The transgenic hydra were provided by Robert Steele, a study co-author and a Professor of Biological Chemistry at the UC Irvine School of Medicine.
|A transgenic hydra with its ectodermal cells (green) and endodermal cells (red) clearly differentiated.|
Image Credit: Callen Hyland, UC San Diego
By using live imaging, the team tracked individual cells during spontaneous mouth openings. The results showed the cells remained next to their surrounding cells—not rearranging themselves—and just stretched and deformed. Mouth opening is propelled by radially oriented muscle fibrils called myonemes located among the ectodermal cells. Because one individual can open consecutively to different amounts, this suggests that the degree of mouth opening is controlled through neuronal signaling. The forces used by the myonemes to open the hydra's mouth was estimated to be a few nanoNewtons.
That myonemes caused mouth opening was confirmed using a muscle relaxant, magnesium chloride. The presence of magnesium chloride blocked myoneme activity and also mouth opening.
Using shape analysis on the cells further confirmed that mouth opening occurs with elastic deformation of the cells around the mouth.
The research team believes the hydra is a simple, accessible in vivo model to study biomechanical dynamics on the cellular and tissue level, the researchers said in the Biophysical Journal article. “We can now use this system to examine more closely two processes that are fundamental to all organisms: tissue formation and patterning,” Collins said in the press release.
|The Hydra's mouth, shown in cross section and top-down view. The radial myonemes control the organism's mouth in a manner very similar to the contraction and dilation of the human pupil. One significant difference between the two systems, however, is that when it's not in use, the Hydra's mouth vanishes entirely.|
Image Credit: Carter, et al. Biophysical Journal